Drive system health monitor

ABSTRACT

A drive system of a gas turbine engine includes a first drive shaft and a second drive shaft operable to rotate within the gas turbine engine, a first sensor operable to detect rotation of the first drive shaft, a second sensor operable to detect rotation of the second drive shaft, and a processing system coupled to the first sensor and the second sensor. The processing system is operable to determine a timing variation based on output of the first sensor and output of the second sensor, determine a torsional deflection between the first drive shaft and the second drive shaft based on the timing variation, and detect a health status of the drive system based on the torsional deflection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/812,644, filed Mar. 9, 2020, which claims the benefit of U.S. patentapplication Ser. No. 15/788,020 filed Oct. 19, 2017, and issued as U.S.Pat. No. 10,590,796, issued Mar. 17, 2020, the disclosures of which areincorporated by reference herein in their entirety.

BACKGROUND

The subject matter disclosed herein generally relates to measurementsystems and, more particularly, to a method and an apparatus for drivesystem health monitoring of a gas turbine engine.

Gas turbine engines typically include a compressor, a combustor, and aturbine, with an annular flow path extending axially through each.Initially, air flows through the compressor where it is compressed orpressurized. The combustor then mixes and ignites the compressed airwith fuel, generating hot combustion gases. These hot combustion gasesare then directed from the combustor to the turbine where power isextracted from the hot gases by causing blades of the turbine to rotate.The rotation also drives rotation of a fan that provides thrust undervarious operating conditions.

Multiple drive shafts may be used to link rotation of various stages ofthe turbine, compressor, and fan. Monitoring systems can be used tomeasure conditions within a gas turbine engine for monitoringdegradation that may lead to a future servicing event, as well asidentify maintenance conditions. Engine vibrations are typicallymonitored using accelerometers that can detect vibrations in one or moreaxis. However, accelerometers may not readily detect all desiredconditions of rotating components that can be monitored for potentialmaintenance events.

BRIEF DESCRIPTION

According to one embodiment a drive system of a gas turbine engineincludes a first drive shaft and a second drive shaft operable to rotatewithin the gas turbine engine, a first sensor operable to detectrotation of the first drive shaft, a second sensor operable to detectrotation of the second drive shaft, a drive gear system coupled to thefirst drive shaft and the second drive shaft, and a processing systemcoupled to the first sensor and the second sensor. The processing systemis operable to determine a timing variation based on output of the firstsensor and output of the second sensor, determine a torsional deflectionbetween the first drive shaft and the second drive shaft based on thetiming variation, and detect a health status of the drive system basedon the torsional deflection. The health status identifies whether afault condition of the drive gear system is detected based on thetorsional deflection.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the processingsystem is operable to perform a frequency domain analysis based onoutput of the first sensor.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the processingsystem is operable to identify a dominant mode as a shaft frequency ofthe first drive shaft and a lower amplitude frequency domain componentas a torsional mode of the first drive shaft.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the shaftfrequency is used to identify an operating mode of the gas turbineengine, and trending of the torsional mode is determined based on theoperating mode of the gas turbine engine.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the processingsystem is operable to perform a frequency domain analysis based onoutput of the second sensor and identify a dominant mode as a shaftfrequency of the second drive shaft and a lower amplitude frequencydomain component as a torsional mode of the second drive shaft.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the shaftfrequency is used to identify an operating mode of the gas turbineengine, and trending of the torsional mode is determined based on theoperating mode of the gas turbine engine.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the processingsystem is operable to track the timing variation between a firstposition indicator associated with the first drive shaft and a secondposition indicator associated with the second drive shaft.

According to another embodiment, a gas turbine engine includes a drivesystem including a first drive shaft operable to drive a fan of the gasturbine engine and a second drive shaft operable to be driven by aturbine of the gas turbine engine. The gas turbine engine also includesa first sensor operable to detect rotation of the first drive shaft, asecond sensor operable to detect rotation of the second drive shaft, anda processing system coupled to the first and second sensors. Theprocessing system is operable to identify an operating mode of the gasturbine engine, determine a trend of a torsional mode of the drivesystem based on the operating mode of the gas turbine engine, and detecta health status of the gas turbine engine based on the torsional mode.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the processingsystem is operable to perform a frequency domain analysis based onoutput of the first sensor and identify a dominant mode as a shaftfrequency of the first drive shaft and a lower amplitude frequencydomain component as a torsional mode of the first drive shaft.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the shaftfrequency is used to identify the operating mode of the gas turbineengine.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the processingsystem is operable to perform a frequency domain analysis based onoutput of the second sensor and identify a dominant mode as a shaftfrequency of the second drive shaft and a lower amplitude frequencydomain component as a torsional mode of the second drive shaft.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where the shaftfrequency is used to identify the operating mode of the gas turbineengine.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where a drive gearsystem is coupled between the first drive shaft and the second driveshaft, and the health status identifies whether a fault condition of thedrive gear system is detected.

According to a further embodiment, a method of monitoring a drive systemin a gas turbine engine is provided. The method includes detectingrotation of a first drive shaft via a first sensor operably coupled to aprocessing system, detecting rotation of a second drive shaft via asecond sensor operably coupled to the processing system, identifying anoperating mode of the gas turbine engine, and determining, by theprocessing system, a trend of a torsional mode of the drive system basedon the operating mode of the gas turbine engine. The method alsoincludes detecting a health status of the drive system based on thetorsional mode.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include performing, by theprocessing system, a frequency domain analysis based on output of thefirst sensor.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include identifying adominant mode as a shaft frequency and a lower amplitude frequencydomain component as a torsional mode of the first drive shaft, where theshaft frequency is used to identify the operating mode of the gasturbine engine.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include performing afrequency domain analysis based on output of the second sensor andidentify a dominant mode as a shaft frequency of the second drive shaftand a lower amplitude frequency domain component as a torsional mode ofthe second drive shaft.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where a timingvariation is tracked based on a first position indicator associated withthe first drive shaft and a second position indicator associated withthe second drive shaft.

In addition to one or more of the features described above or below, oras an alternative, further embodiments may include where a drive gearsystem is coupled between the first drive shaft and the second driveshaft, and the health status identifies whether a fault condition of thedrive gear system is detected.

A technical effect of the apparatus, systems and methods is achieved byusing one or more speed sensors to determine torsional modes of a drivesystem in a gas turbine engine as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The following descriptions should not be considered limiting in any way.With reference to the accompanying drawings, like elements are numberedalike:

FIG. 1 is a partial cross-sectional illustration of a gas turbineengine, in accordance with an embodiment of the disclosure;

FIG. 2 is a schematic illustration of a drive system of a gas turbineengine, in accordance with an embodiment of the disclosure;

FIG. 3 is a frequency response plot of speed sensor data, in accordancewith an embodiment of the disclosure;

FIG. 4 is a timing diagram of phonic wheel pulse trains, in accordancewith an embodiment of the disclosure;

FIG. 5 is a block diagram of a processing system, in accordance with anembodiment of the disclosure; and

FIG. 6 is a flow chart illustrating a method, in accordance with anembodiment of the disclosure.

DETAILED DESCRIPTION

A detailed description of one or more embodiments of the disclosedapparatus and method are presented herein by way of exemplification andnot limitation with reference to the Figures.

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. Alternative engines mightinclude an augmentor section (not shown) among other systems orfeatures. The fan section 22 drives air along a bypass flow path B in abypass duct, while the compressor section 24 drives air along a coreflow path C for compression and communication into the combustor section26 then expansion through the turbine section 28. Although depicted as atwo-spool turbofan gas turbine engine in the disclosed non-limitingembodiment, it should be understood that the concepts described hereinare not limited to use with two-spool turbofans as the teachings may beapplied to other types of turbine engines including three-spoolarchitectures.

The exemplary engine 20 generally includes a low speed spool 30 and ahigh speed spool 32 mounted for rotation about an engine centrallongitudinal axis A relative to an engine static structure 36 viaseveral bearing systems 38. It should be understood that various bearingsystems 38 at various locations may alternatively or additionally beprovided, and the location of bearing systems 38 may be varied asappropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects a fan 42, a low pressure compressor 44 and a low pressureturbine 46. The inner shaft 40 is connected to the fan 42 through aspeed change mechanism, which in exemplary gas turbine engine 20 isillustrated as a geared architecture 48 to drive the fan 42 at a lowerspeed than the low speed spool 30. The high speed spool 32 includes anouter shaft 50 that interconnects a high pressure compressor 52 and highpressure turbine 54. A combustor 56 is arranged in exemplary gas turbine20 between the high pressure compressor 52 and the high pressure turbine54. An engine static structure 36 is arranged generally between the highpressure turbine 54 and the low pressure turbine 46. The engine staticstructure 36 further supports bearing systems 38 in the turbine section28. The inner shaft 40 and the outer shaft 50 are concentric and rotatevia bearing systems 38 about the engine central longitudinal axis Awhich is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The turbines 46, 54 rotationally drive therespective low speed spool 30 and high speed spool 32 in response to theexpansion. It will be appreciated that each of the positions of the fansection 22, compressor section 24, combustor section 26, turbine section28, and fan drive gear system 48 may be varied. For example, gear system48 may be located aft of combustor section 26 or even aft of turbinesection 28, and fan section 22 may be positioned forward or aft of thelocation of gear system 48.

The engine 20 in one example is a high-bypass geared aircraft engine. Ina further example, the engine 20 bypass ratio is greater than about six(6), with an example embodiment being greater than about ten (10), thegeared architecture 48 is an epicyclic gear train, such as a planetarygear system or other gear system, with a gear reduction ratio of greaterthan about 2.3 and the low pressure turbine 46 has a pressure ratio thatis greater than about five. In one disclosed embodiment, the engine 20bypass ratio is greater than about ten (10:1), the fan diameter issignificantly larger than that of the low pressure compressor 44, andthe low pressure turbine 46 has a pressure ratio that is greater thanabout five 5:1. Low pressure turbine 46 pressure ratio is pressuremeasured prior to inlet of low pressure turbine 46 as related to thepressure at the outlet of the low pressure turbine 46 prior to anexhaust nozzle. The geared architecture 48 may be an epicycle geartrain, such as a planetary gear system or other gear system, with a gearreduction ratio of greater than about 2.3:1. It should be understood,however, that the above parameters are only exemplary of one embodimentof a geared architecture engine and that the present disclosure isapplicable to other gas turbine engines including direct driveturbofans.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet (10,688 meters). The flight condition of 0.8 Mach and35,000 ft (10,688 meters), with the engine at its best fuelconsumption—also known as “bucket cruise Thrust Specific FuelConsumption (‘TSFC’)”—is the industry standard parameter of lbm of fuelbeing burned divided by lbf of thrust the engine produces at thatminimum point. “Low fan pressure ratio” is the pressure ratio across thefan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The lowfan pressure ratio as disclosed herein according to one non-limitingembodiment is less than about 1.45. “Low corrected fan tip speed” is theactual fan tip speed in ft/sec divided by an industry standardtemperature correction of [(Tram ° R)/(518.7° R)]{circumflex over( )}0.5. The “Low corrected fan tip speed” as disclosed herein accordingto one non-limiting embodiment is less than about 1150 ft/second (350.5m/sec).

Referring now to FIG. 2, a drive system 100 of the gas turbine engine 20of FIG. 1 is depicted. In the example of FIG. 2, the drive system 100includes a first drive shaft 102 a coupled between the fan 42 and thefan drive gear system 48 (also referred to as a drive gear system 48).The drive system 100 also includes a second drive shaft 102 b coupledbetween the low pressure turbine 46 and the fan drive gear system 48such that at least one drive shaft 102 a, 102 b is operable to rotatewithin the gas turbine engine 20. A first phonic wheel 106 a is coupledto the first drive shaft 102 a, and a second phonic wheel 106 b iscoupled to the second drive shaft 102 b. A first speed sensor 108 a isoperable to detect rotation of the first phonic wheel 106 a indicativeof rotation of the first drive shaft 102 a. Similarly, a second speedsensor 108 b is operable to detect rotation of the second phonic wheel106 b indicative of rotation of the second drive shaft 102 b. The firstphonic wheel 106 a may also be referred to as a fan phonic wheel,including a number of teeth 110 a. Similarly, the second phonic wheel106 b may also be referred to as a low pressure turbine phonic wheel,including a number of teeth 110 b. In some embodiments, the number ofteeth 110 a of the first phonic wheel 106 a is the same as the number ofteeth 110 b of the second phonic wheel 106 b. In other embodiments, thenumber of teeth 110 a of the first phonic wheel 106 a is different thanthe number of teeth 110 b of the second phonic wheel 106 b.

The teeth 110 a of the first phonic wheel 106 a may induce a detectablesignal at the first speed sensor 108 a as each tooth passes in closephysical proximity to the first speed sensor 108 a (e.g., throughelectro-magnetic communication). Similarly, teeth 110 b of the secondphonic wheel 106 b may induce a detectable signal at the second speedsensor 108 b as each tooth passes in close physical proximity to thesecond speed sensor 108 b. The teeth 110 a of the first phonic wheel 106a and the teeth 110 b of the second phonic wheel 106 b can besubstantially regularly spaced. However, one of the teeth 110 a of thefirst phonic wheel 106 a and one of the teeth 110 b of the second phonicwheel 106 b can be physically offset or be physically extended to createa detectable position indicator for each of the first and second phonicwheel 106 a, 106 b, e.g., a once-per-revolution indicator.

In the example of FIG. 2, the first drive shaft 102 a and the seconddrive shaft 102 b are mechanically coupled through the fan drive gearsystem 48. Rotation of low pressure turbine 46 drives rotation of thesecond drive shaft 102 b and drives rotation of the first drive shaft102 a through the fan drive gear system 48 to rotate the fan 42. A gearratio of the fan drive gear system 48 can result in the first driveshaft 102 a rotating at a slower speed than the second drive shaft 102b. Thus, a phonic wheel pulse train induced by rotation of the firstphonic wheel 106 a and detected by the first speed sensor 108 a cantransition slower than a phonic wheel pulse train induced by rotation ofthe second phonic wheel 106 b and detected by the second speed sensor108 b as both the first and second drive shafts 102 a, 102 b rotate.

In embodiments, a processing system 112 is coupled to at least one ofthe first and second speed sensors 108 a, 108 b. In the example of FIG.2, the processing system 112 is coupled to both the first and secondspeed sensors 108 a, 108 b. The processing system 112 is operable todetect one or more phonic wheel pulse trains, determine a torsional modebased on the one or more phonic wheel pulse trains, and record one ormore trends of the torsional mode indicative of a health status of thedrive system 100. The processing system 112 may use one or more signalprocessing techniques to determine torsional modes based on speed sensorsignals from the first speed sensor 108 a and/or the second speed sensor108 b.

As an example, the processing system 112 can perform individual shafttorsional analysis on a per shaft basis by separately analyzing datafrom each of the first speed sensor 108 a and the second speed sensor108 b. The processing system 112 can perform a frequency domain analysisof a phonic wheel pulse train from the first speed sensor 108 a or thesecond speed sensor 108 b and identify a dominant mode 302 as a shaftfrequency and a lower amplitude frequency domain component 304 as atorsional mode as depicted in the frequency response plot 300 of FIG. 3.The dominant mode 302 tracks with respect to rotational speed of therespective drive shaft (e.g., the first drive shaft 102 a or the seconddrive shaft 102 b) being monitored. The torsional mode has a loweramplitude than the dominant mode 302 and does not directly correlate todrive shaft rotational speed. The torsional mode represents a resonancedue to oscillations in shaft loading. Frequency content in speed sensorsignals, such as that depicted in frequency response plot 300, can beproduced using a Fourier transform, a wavelet-based transform, or otherknown techniques. The same or similar frequency analysis can beperformed with respect to the first drive shaft 102 a and the seconddrive shaft 102 b.

The shaft frequency can be used to identify an operating mode of the gasturbine engine 20, and trending of the torsional mode can be performedbased on the operating mode of the gas turbine engine 20. For example, aspeed range and transition sequence between speed ranges (as well as oneor more other parameters) can be used to identify whether the gasturbine engine 20 is operating at ground idle, flight idle, max cruise,take-off, max power, or another known operating mode. The processingsystem 112 can collect a buffer of speed sensor values over a collectionperiod for each of the first and second speed sensors 108 a, 108 b. Thebuffered speed sensor data can be analyzed to determine whether thecollection period was substantially steady state and did not include anoperating mode transition, for instance, the speed did not vary by morethan a predetermined steady state threshold. If the collected speed datawas determined to be steady state, the torsional mode can be determined,for instance, using a frequency domain transform as previouslydescribed. The torsional mode can be tracked with respect to theidentified operating mode of the gas turbine engine 20. Trending ofchanges in the torsional mode can be performed on an operating modebasis to determine whether changes are indicative of increased shaftfatigue. For example, a rate of change of the torsional mode above achange threshold may be used to trigger an indicator, such as a healthstatus. The health status can be set to initiate one or more actions,such as an inspection event, a maintenance event, additional monitoringevents, and/or other events internal or external to the gas turbineengine 20. Further, unlike typical accelerometer based vibrationmonitoring systems, the torsional mode is not located at the shaftfrequency where the dominant mode 302 is located and can be independentof the precise value of the shaft frequency.

Although the frequency response plot 300 is depicted with only twofrequency components, it will be understood that additional frequencycomponents (not depicted) may also be captured in the frequency responseplot 300. For example, there may be spectral spreading and/or harmonicsdepending upon filtering and alignment between spectral bins of thefrequency domain transform and the actual frequencies observed.

As another example, the processing system 112 can observe a first phonicwheel pulse train 402 and a second phonic wheel pulse train 404 andtrack a timing variation between a first position indicator 406 of thefirst phonic wheel 106 a and a second position indicator 408 of thesecond phonic wheel 106 b as depicted in the timing diagram 400 of FIG.4. In the example of FIG. 4, the first position indicator 406 is arising edge of an offset tooth of the first phonic wheel 106 a, and thesecond position indicator 408 is a rising edge of an offset tooth of thesecond phonic wheel 106 b. A time difference 410 between the first andsecond position indicator 406, 408 can be tracked for variationsindicative of torsional resonance through the fan drive gear system 48of FIG. 2. The processing system 112 can determine a torsionaldeflection between the first drive shaft 102 a with respect to thesecond drive shaft 102 b based on the timing variation between the firstposition indicator 406 of the first phonic wheel 106 a and the secondposition indicator 408 of the second phonic wheel 106 b. As the gearratio of the fan drive gear system 48 results in different rotationalspeeds of the first and second phonic wheel 106 a, 106 b, multiplerevolutions of the first and second phonic wheel 106 a, 106 b can betracked in groups to account for relative positional variations. Forexample, if the gear ratio of the fan drive gear system 48 results in a2.3:1 speed reduction, then 10 revolutions of the first drive shaft 102a corresponds with 23 revolutions of the second drive shaft 102 b, andthe expected alignment positions can be compared to the observedalignment positions. However, the expected alignment positions need notbe used, as the pattern of changes in the time difference 410 at aparticular speed can be tracked to monitor torsional deflection. Similarto the frequency domain example of FIG. 3, the torsional deflection canbe tracked with respect to the operating mode of the gas turbine engine20, and the health status can be set to identify whether a faultcondition of the drive gear system 48 is detected based on the torsionaldeflection.

Referring now to FIG. 5, an example of the processing system 112 isshown in greater detail. The processing system 112 includes a memory 502which can store executable instructions and/or data associated withcontrol and/or diagnostic/prognostic systems of the gas turbine engine20 of FIG. 1. The executable instructions can be stored or organized inany manner and at any level of abstraction, such as in connection withone or more applications, processes, routines, procedures, methods, etc.As an example, at least a portion of the instructions are shown in FIG.5 as being associated with a control program 504.

Further, as noted, the memory 502 may store data 506. The data 506 mayinclude, but is not limited to, values to support detecting theoperating mode of the gas turbine engine 20, commands for variousactuators, lookup tables, sensor data, communication data, or any othertype(s) of data as will be appreciated by those of skill in the art. Oneor more speed sensor buffer 516 and/or torsional mode trends 518 can bestored in the memory 502 and may be part of or separate from the data506. The instructions stored in the memory 502 may be executed by one ormore processors, such as a processor 508. The processor 508 may beoperative on the data 506, speed sensor buffer 516, and/or torsionalmode trends 518.

The processor 508 can be any type or combination of computer processors,such as a microprocessor, microcontroller, digital signal processor,application specific integrated circuit, programmable logic device,and/or field programmable gate array. The memory 502 is an example of anon-transitory computer readable storage medium tangibly embodied in oroperably connected to the processing system 112 including executableinstructions stored therein, for instance, as firmware.

The processor 508, as shown, is coupled to one or more input/output(I/O) devices through an I/O interface 510. For example, the I/Ointerface 510 can be operable to receive speed sensor signals 512 a, 512b from the first and second speed sensors 108 a, 108 b of FIG. 2. Theprocessor 508 can also communicate with one or more other systems (notdepicted) using a communication interface 514 to send and receivemessages on one or more communication buses 515, which may includetransmitting a health status 517 based on torsional values captured inthe torsional mode trends 518. The processing system 112 may furtherinclude other features or components as known in the art. For example,the processing system 112 may include one or more transceivers and/ordevices configured to transmit and/or receive information or data fromsources external to the processing system 112. For example, in someembodiments, the processing system 112 may be configured to receiveinformation over a network (wired or wireless) or through a cable orwireless connection with one or more devices remote from the processingsystem 112 via the communication interface 514. The information receivedcan stored in the memory 502 (e.g., as data 506) and/or may be processedand/or employed by one or more programs or applications (e.g., program504) and/or the processor 508.

The processing system 112 can also include one or more counters 520and/or timers 522. The counters 520 can be used, for example, to trackthe teeth 110 a, 110 b of the first and second phonic wheel 106 a, 106 band assist in determining the location of the first and second positionindicator 406, 408, for instance, based on tooth-to-tooth timingvariations observed via timers 522. The timers 522 can also supportobservations of the time difference 410 between the first and secondposition indicator 406, 408.

Although the processing system 112 is depicted as a single system, itwill be understood the portions of the processing system 112 can bedistributed between multiple processing circuits, including multipleinstances of the processor 508, memory 502, and the like.

Referring now to FIG. 6 with continued reference to FIGS. 1-5. FIG. 6 isa flow chart illustrating a method 500 for health monitoring of thedrive system 100 in a gas turbine engine 20, in accordance with anembodiment. At block 602, a phonic wheel pulse train indicative ofrotation of at least one drive shaft is detected via a speed sensoroperably coupled to processing system 112, such as the first phonicwheel pulse train 402 of the first drive shaft 102 a detected by thefirst speed sensor 108 a or the second phonic wheel pulse train 404 ofthe second drive shaft 102 b detected by the second speed sensor 108 b.

At block 604, the processing system 112 determines a torsional mode ofthe at least one drive shaft based on the phonic wheel pulse train. Theprocessing system 112 may perform a frequency domain analysis of the oneor more phonic wheel pulse trains as previously described in referenceto FIG. 3. The frequency domain analysis can include identifying adominant mode 302 as a shaft frequency and a lower amplitude frequencydomain component 304 as the torsional mode. The shaft frequency can beused to identify an operating mode of the gas turbine engine 20. Forexample, the operating mode can be identified as one of: ground idle,flight idle, max cruise, take-off, max power, and any other flightcondition known to one of skill in the art. In some embodiments, theprocessing system 112 can track a timing variation between a firstposition indicator 406 of the first phonic wheel 106 a on the firstdrive shaft 102 a and a second position indicator 408 of the secondphonic wheel 106 b on the second drive shaft 102 b. A torsionaldeflection between the first drive shaft 102 a with respect to thesecond drive shaft 102 b can be determined based on the timing variation(e.g., changes in time difference 410) between the first positionindicator 406 of the first phonic wheel 106 a and the second positionindicator 408 of the second phonic wheel 106 b.

At block 606, one or more trends of the torsional mode indicative of ahealth status 517 of the drive system 100 are recorded, for instance, intorsional mode trends 518. As previously described, trending of thetorsional mode can determined based on the operating mode of the gasturbine engine 20. The health status 517 identifies whether a faultcondition of the drive gear system 48 can be detected based on thetorsional deflection.

While the above description has described the flow process of FIG. 6 ina particular order, it should be appreciated that unless otherwisespecifically required in the attached claims that the ordering of thesteps may be varied.

The term “about” is intended to include the degree of error associatedwith measurement of the particular quantity based upon the equipmentavailable at the time of filing the application.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the presentdisclosure. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,element components, and/or groups thereof.

While the present disclosure has been described with reference to anexemplary embodiment or embodiments, it will be understood by thoseskilled in the art that various changes may be made and equivalents maybe substituted for elements thereof without departing from the scope ofthe present disclosure. In addition, many modifications may be made toadapt a particular situation or material to the teachings of the presentdisclosure without departing from the essential scope thereof.Therefore, it is intended that the present disclosure not be limited tothe particular embodiment disclosed as the best mode contemplated forcarrying out this present disclosure, but that the present disclosurewill include all embodiments falling within the scope of the claims.

What is claimed is:
 1. A drive system of a gas turbine engine, the drivesystem comprising: a first drive shaft and a second drive shaft operableto rotate within the gas turbine engine; a first sensor operable todetect rotation of the first drive shaft; a second sensor operable todetect rotation of the second drive shaft; a drive gear system coupledto the first drive shaft and the second drive shaft; and a processingsystem coupled to the first sensor and the second sensor, the processingsystem operable to: determine a timing variation based on output of thefirst sensor and output of the second sensor; determine a torsionaldeflection between the first drive shaft and the second drive shaftbased on the timing variation; and detect a health status of the drivesystem based on the torsional deflection, wherein the health statusidentifies whether a fault condition of the drive gear system isdetected based on the torsional deflection.
 2. The drive system of claim1, wherein the processing system is operable to perform a frequencydomain analysis based on output of the first sensor.
 3. The drive systemof claim 2, wherein the processing system is operable to identify adominant mode as a shaft frequency of the first drive shaft and a loweramplitude frequency domain component as a torsional mode of the firstdrive shaft.
 4. The drive system of claim 3, wherein the shaft frequencyis used to identify an operating mode of the gas turbine engine, andtrending of the torsional mode is determined based on the operating modeof the gas turbine engine.
 5. The drive system of claim 1, wherein theprocessing system is operable to perform a frequency domain analysisbased on output of the second sensor and identify a dominant mode as ashaft frequency of the second drive shaft and a lower amplitudefrequency domain component as a torsional mode of the second driveshaft.
 6. The drive system of claim 5, wherein the shaft frequency isused to identify an operating mode of the gas turbine engine, andtrending of the torsional mode is determined based on the operating modeof the gas turbine engine.
 7. The drive system of claim 1, wherein theprocessing system is operable to track the timing variation between afirst position indicator associated with the first drive shaft and asecond position indicator associated with the second drive shaft.
 8. Agas turbine engine comprising: a drive system comprising: a first driveshaft operable to drive a fan of the gas turbine engine; and a seconddrive shaft operable to be driven by a turbine of the gas turbineengine; a first sensor operable to detect rotation of the first driveshaft; a second sensor operable to detect rotation of the second driveshaft; and a processing system coupled to the first and second sensors,the processing system operable to: identify an operating mode of the gasturbine engine; determine a trend of a torsional mode of the drivesystem based on the operating mode of the gas turbine engine; and detecta health status of the gas turbine engine based on the torsional mode.9. The gas turbine engine of claim 8, wherein the processing system isoperable to perform a frequency domain analysis based on output of thefirst sensor and identify a dominant mode as a shaft frequency of thefirst drive shaft and a lower amplitude frequency domain component as atorsional mode of the first drive shaft.
 10. The gas turbine engine ofclaim 9, wherein the shaft frequency is used to identify the operatingmode of the gas turbine engine.
 11. The gas turbine engine of claim 8,wherein the processing system is operable to perform a frequency domainanalysis based on output of the second sensor and identify a dominantmode as a shaft frequency of the second drive shaft and a loweramplitude frequency domain component as a torsional mode of the seconddrive shaft.
 12. The gas turbine engine of claim 11, wherein the shaftfrequency is used to identify the operating mode of the gas turbineengine.
 13. The gas turbine engine of claim 8, wherein a drive gearsystem is coupled between the first drive shaft and the second driveshaft, and the health status identifies whether a fault condition of thedrive gear system is detected.
 14. A method of monitoring a drive systemin a gas turbine engine, the method comprising: detecting rotation of afirst drive shaft via a first sensor operably coupled to a processingsystem; detecting rotation of a second drive shaft via a second sensoroperably coupled to the processing system; identifying an operating modeof the gas turbine engine; determining, by the processing system, atrend of a torsional mode of the drive system based on the operatingmode of the gas turbine engine; and detecting a health status of thedrive system based on the torsional mode.
 15. The method of claim 14,further comprising: performing, by the processing system, a frequencydomain analysis based on output of the first sensor.
 16. The method ofclaim 15, further comprising: identifying a dominant mode as a shaftfrequency and a lower amplitude frequency domain component as atorsional mode of the first drive shaft, wherein the shaft frequency isused to identify the operating mode of the gas turbine engine.
 17. Themethod of claim 14, further comprising: performing a frequency domainanalysis based on output of the second sensor and identify a dominantmode as a shaft frequency of the second drive shaft and a loweramplitude frequency domain component as a torsional mode of the seconddrive shaft.
 18. The method of claim 17, wherein the shaft frequency isused to identify the operating mode of the gas turbine engine.
 19. Themethod of claim 14, wherein a timing variation is tracked based on afirst position indicator associated with the first drive shaft and asecond position indicator associated with the second drive shaft. 20.The method of claim 14, wherein a drive gear system is coupled betweenthe first drive shaft and the second drive shaft, and the health statusidentifies whether a fault condition of the drive gear system isdetected.